For transmission purposes, an information-carrying signal is generally put into electrical form, into the form of a radiated electromagnetic wave, or into the form of a guided light wave, with guided light waves generally being conveyed within a monomode optical fiber. Such a fiber conventionally comprises a core conveying the signal and optical cladding integral with the core, surrounding the core, and having a lower refractive index, thereby giving rise to an "evanescent" wave which also conveys the signal, but which is normally detectable only at a very short distance from the core.
The optical cladding is itself surrounded by an outer sheath which is different in nature and which provides mechanical and chemical protection.
Given the numerous advantages of monomode fibers (low loss, very high data rate, immunity from electromagnetic interference, ...), their use is becoming more and more widespread, including use in local networks. In addition, for information protection purposes, the guided optical nature of the signal makes an intrusion more difficult than when using conventional electric wires or when using a radiated electromagnetic link. That is why the present invention applies to said guided optical nature, and more precisely to guidance via a monomode fiber.
However, in local processing of a connection by melting together two fibers, it has recently been discovered that information can be tapped by means of curves or micro-curves which cause the phase velocity of a mode guided in the core to become less than the velocity of plane waves that may propagate in the outer sheath. If the fiber is curved, the phase velocity increases in proportion to distance from the center of curvature, and beyond a critical degree of curvature it exceeds the velocity of plane waves. Under such circumstances the evanescent wave is easily sensed.
Abrasive methods make it possible to come closer to the core and may also be used for tapping information since couplers for micromode fibers are based on this principle.
The object of the present invention is to provide effective protection for information being transmitted in a monomode fiber.